The present invention relates to a method for the non-destructive testing of welding joints between sheets of metal and bolts welded onto the same, in particular stud bolts welded on using the stroke ignition method. Such bolts having various geometrical shapes which are welded on using the fast-welding method are used in a wide variety of applications, e.g. as fastening elements for body-making in the automobile industry. Fast-welding methods are mostly automatic and are widely used in industry. Up to now, quality testing of the weld joints produced automatically is still problematic. Such tests are necessary in many cases as safety-relevant joints are often implemented using welded-on stud bolts.
Until now, the only type of testing methods implemented in practice are purely mechanical ones. These are based on determining mechanical characteristics of the weld joints to be tested, in the present case especially the tear-off, bending or turn-off momentum of the welded-on bolt. However, one of the problems in connection with the mechanical testing methods is that an earlier damage and thus a possible later fault of the weld joint tested cannot be excluded.
Known non-destructive testing methods relying on the use of ultrasonic sound are essentially based on detecting the attenuation of ultrasonic signals spreading in the direction of the longitudinal axis of the welded-on bolt during the reflection on and/or the transmission through the weld pool of the weld joint to be tested. Until now, only longitudinal waves were used in such tests. Such testing methods were implemented by means of test heads emitting sound waves in a perpendicular direction in single-head operation. Because of the generally uneven surface of the bolt head, the longitudinal waves were introduced from the rear side of the even sheet metal surface. Thus, pulse-reflection methods operating in the direction of the bolt""s longitudinal axis were used.
Such testing methods based on ultrasonic sound have not yet reached a degree of reliability justifying their use for testing weld joints between bolts and sheet metal as a standard. These ultrasonic sound testing methods exhibited an unsatisfactory correlation between the results of the ultrasonic test and those of the mechanical testing methods.
This object is achieved by a method comprising the features specified in claim 1 and by an apparatus comprising the features specified in claim 10.
The basic idea of the method according to the present invention is to distinguish intact weld joints from faulty weld joints on the basis of the weld pool diameter. A weld pool represents that part of a sheet metal in which the material structure has changed due to the welding operation in comparison to the non-rolled sheet metal. Thus, the method according to the present invention is based on the detection of the weld pool diameter. The most decisive criterion is whether a minimum diameter for a predetermined weld joint has been achieved or not.
Of course, this minimum diameter of the weld pool depends upon the specific parameters of the weld joint to be tested. The factors requiring particular consideration include the geometry of the welded-on stud bolt, the geometry of the sheet metal, the material characteristics of the stud bolt and the sheet metal as well as the specific characteristics of the welding method used. With respect to a predetermined combination of stud bolt and sheet metal and a defined welding method, it is possible, for example within the framework of preliminary tests, to determine the diameter of the weld pool using the non-destructive testing method according to the present invention. Thereafter, the known mechanical testing methods may be used to determine from which diameter of the weld pool the weld joints can be regarded as intact. In this way, a standard minimum radius of the weld pool can be determined.
The testing method according to the present invention used for preferably automated testing of such weld joints, is essentially based on examining the weld joint to be tested to find out whether the standard minimum radius of the weld pool determined during the preliminary tests has been reached in the actual case or not. In this way, a secure classification of the weld joint to be tested is possible.
The method according to the present invention is based on a directed spreading of ultrasonic signals in the sheet metal onto which a stud bolt has been welded. Here, the direction of sound propagation of the ultrasonic signals comprises at least one component directed towards the extension direction of the sheet metal. Thus, the ultrasonic signals can be made to penetrate into the sheet metal at a first point A and coupled out of the sheet metal at a second point B with the points A and B being spaced apart from each other. The ultrasonic penetration is performed by an ultrasonic source sending sound signals in an oblique direction into the coupling-in point A. The ultrasonic signals are coupled out at the coupling-out point B using an ultrasonic receiver being specifically sensitive for ultrasonic signals being obliquely incident from the direction of the coupling-in point A. In particular, the ultrasonic receiver may have essentially the same ultrasonic acoustic characteristics as the ultrasonic transmitter. In general, the acoustic path extending from the coupling-In point A to the coupling-out point B tends to comprise a plurality of reflections at the surfaces of the sheet metal. Preferably, the distance between the coupling-in point A and the coupling-out point B Is chosen so that, in the light of the given material characteristics of the sheet metal and the ultrasonic parameters, the undisturbed acoustic path of the ultrasonic signals penetrating into the sheet metal at the coupling-In point A essentially hits the coupling-out point B.
For testing a weld joint using the method according to the present invention, the coupling-in point A and the coupling-out point B are placed on the sheet metal so that the connecting line between both points A and B passes at least through the edge area of the weld joint to be tested. The minimum distance of the connecting line between both points from the centre of the contact area is indicated by the reference sign d. By displacing in parallel the connecting line between A and B, it is now possible in principle for the ultrasonic signals to scan the entire weld pool of the weld joint to be tested.
The method of the present invention is based on the fact that the weld pool of the weld joint to be tested due to its modified material structure has other ultrasonic acoustic characteristics than the material structure of the sheet metal which in general has been manufactured in a rolling process. These modified ultrasonic acoustic characteristics of the weld pool cause a reduced transmission between the coupling-in point A and the coupling-out point B if the welded structure were located between these two points, i.e. if the path of the ultrasonic signals passes through the weld pool.
When the intensity of the ultrasonic signals transmitted from point A to point B is recorded location-related as a function of the distance d, it is possible to determine the size of the weld pool from the shape of the resulting graph. The location-related recording of the intensity of the ultrasonic signals can be used in particular within the framework of the preliminary tests described for determining the diameter of weld joints classified as intact. In this case, it is preferred with regard to a given weld joint to determine a standard minimum radius of the weld pool of the weld joint to be tested and to classify this weld joint to be tested as intact if said minimum radius is exceeded. Should the standard minimum radius not be reached, the respective weld joint to be tested will be classified as faulty. During the practical test, it is possible for example to use a standardised signal level on the flange of the resulting graph of ultrasonic intensity.
In a first embodiment of the method according to the present invention, which can be preferably used within the framework of automated manufacturing and testing methods, the attenuation of ultrasonic signals moving at a distance x from the centre of the weld joint to be tested from point A to point B. This minimum distance x of the acoustic path is equivalent to the minimum distance d between the connecting line A-B and the centre of the contact area indicated by the reference sign d. Here, this distance d is chosen so that the connecting line between A and B extends on or within the standard minimum radius of the weld pool which is regarded as a minimum requirement for classifying the weld joint as intact. In particular, d can be chosen exactly so that the acoustic path of the ultrasonic signals does not scan the weld pool of the weld joint to be tested if this weld pool does not reach the standard minimum radius, i.e. if the weld joint has to be classified as faulty.
Several developments can be used to further enhance the sensitivity of the method according to the present invention. A first sensitivity enhancement of the method according to the present invention is achieved when transversally polarised ultrasonic signals are used in the method according to the present invention. In contrast to the longitudinal waves used in the non-destructive ultrasonic testing methods of the prior art, transversally polarised ultrasonic waves offer a significantly enhanced sensitivity with respect to the material structure of the material in which they spread. In particular, transversally polarised ultrasonic waves undergo a significantly larger attenuation in the more coarse grained material structure of the weld pool than longitudinal waves. Thus, in an advantageous development of the method according to the present invention, mainly transversal waves are excited within the sheet metal. It was found to be especially advantageous when with regard to the ultrasonic capacity at least 75% of the ultrasonic signals generated in the sheet metal are transversally polarised. It is especially advantageous when with regard to the ultrasonic capacity more than 90% of the ultrasonic signals penetrating into the sheet metal are transversally polarised. From the viewpoint of the measuring technology, it is highly advantageous to generate a proportion as large as possible of transversally polarised ultrasonic waves in the sheet metal as this on the one hand principally enhances the sensitivity of the measuring method and on the other hand significantly simplifies the measuring technological evaluation of the ultrasonic signals received at the coupling-out point B. Therefore, any optimisation of the method according to the present invention will always aim at sending a proportion as large as possible of transversally polarised ultrasonic waves into the sheet metal.
This can be achieved advantageously by adapting the penetration angle of the ultrasonic transmitter to the material characteristics of the sheet metal and by adjusting the characteristic parameters of the ultrasonic beam transmitted such as frequency. If an appropriate penetration angle is selected, it is possible to excite only transversal waves in the sheet metal. This phenomenon has been known since long and can be studied in the relevant technological literature dealing with the basic features of ultrasonic waves. It allows in particular to achieve a complete conversion of the generally longitudinally polarised ultrasonic waves transmitted by the ultrasonic transmitter to transversally polarised ultrasonic waves in the sheet metal.
Another development of the method according to the present invention is based on the perception that in case of an intact weld joint it is possible to couple the ultrasonic signals spreading in the sheet metal from A to B into the welded-on bolt. Such coupling-in of the ultrasonic signals into the bolt results in a further attenuation of the ultrasonic signals transmitted through the weld pool, i.e. a further decrease in signal intensity at the coupling-out point B. In order to make the best use of this effect, it is necessary to adjust the acoustic path of the ultrasonic signals in the sheet metal so that the geometric acoustic path as precisely as possible hits the contact area in which the bolt has been welded onto the sheet metal. This can be accomplished by having the ultrasonic signals experience a reflection at this point of the surface of the sheet metal. If the weld joint between the sheet metal and the bolt were properly executed, a particularly high coupling-in level of the ultrasonic signals into the stud bolt is achieved.
In the so-called near field, the directed ultrasonic signals sent by the ultrasonic transmitter into the sheet metal comprise a focus point in which the acoustic pressure has a global maximum. This global maximum is connected with a minimum diameter of the directed ultrasonic beam. Thus, a particularly high location-related resolution of the method according to the present invention can be achieved when the weld pool of the weld joint to be tested is scanned essentially by the focus of the directed ultrasonic beam. Depending upon the material characteristics of the sheet metal and the properties of the ultrasonic transmitter used, in particular its vibration frequency, a focus diameter of one millimetre or less can be achieved. Thus, the diameter of the weld pool can easily be determined with comparable accuracy using the method according to the present invention if the length of the acoustic path between the coupling-in point A and the contact area of the weld joint to be tested is approximately equal to one near field length.
The apparatus according to the present invention is specifically designed so as to apply the method according to the present invention. It comprises an ultrasonic transmitter for transmitting directed ultrasonic signals into the sheet metal at a coupling-in point A and an ultrasonic receiver to couple out, at a coupling-out point B, the ultrasonic signals which were transmitted by the ultrasonic transmitter into the sheet metal and further transmitted through it. Here, the ultrasonic transmitter is designed so as to transmit the signals in an oblique direction. The ultrasonic receiver is arranged relative to the ultrasonic transmitter so that it is located on the acoustic path of the ultrasonic signals in case of an undisturbed spreading of the ultrasonic signals in the metal sheet, i.e. in the sheet metal onto which no bolt has been welded. Here, the ultrasonic receiver is sensitive to oblique incident ultrasonic signals. In particular, the ultrasonic receiver has the same ultrasonic acoustic properties as the ultrasonic transmitter. Further, a spacing device is provided which is designed so as to set a defined distance d from the connecting line between the coupling-in point A end the coupling-out point B to the centre of the contact area of the weld joint to be tested.
When the spacing device is designed so as to select a fixed distance d, said distance d is to be preferably selected shorter than the standard minimum radius of the weld pool of a weld joint to be classified as faultless. This selection of the distance d ensures that the acoustic path of the ultrasonic signals always passes through the weld pool of an intact weld joint.
Further, particular advantages are achieved when the distance d is selected to be longer than the standard radius of the weld pool of a weld joint to be classified as faulty, which radius has also been determined during the preliminary tests.
A combination of the two features last mentioned allows to implement an apparatus the ultrasonic signals of which pass through the weld pool of an intact weld joint while not contacting the weld pool of a faulty weld joint. This course of the acoustic path in the sheet metal allows a secure classification of the weld joint to be tested on the basis of the attenuation occurring between the coupling-in point A and the coupling-out point B, for example compared with undisturbed signal spreading in a sheet metal not comprising a welded-on bolt.
In another advantageous embodiment of the apparatus according to the present invention, the spacing device is designed so that the distance d may be varied. It is especially advantageous when the spacing device is designed so that the entire weld pool of the weld joint to be tested can be scanned by the ultrasonic signals, i.e. that the acoustic path of the ultrasonic signals between the coupling-in point A and the coupling-out point B can be displaced in parallel by varying the distance d so as to scan the entire weld pool.